If you want fresh/live Cornish lobster they cost around £35 a kilo. If you want an air freighted, immature, frozen lobster popsicle with about 100g of meat, then you can go to Lidl.
The story about lobsters is that they were a bycatch - nobody wanted to eat them as they were considered a sea 'insect' . Apparently once refrigeration improved, fishmongers began selling them to naïve landlubbers, who in turn realised they tasted great with butter. They cost about £35 a kilo...
There are plenty of examples of 'peasant' food becoming gourmet littered throughout culinary history- lobster , oyster, paella to name but a few, but a ploughmans was a creation of the Cheese Marketing board or some such. I've no doubts the farm workers would have had bread and cheese for lunch...